database library would be nice for mysql, mariadb and postgresql containers

[phracek]

It would be nice to have a database library, like test-database.sh which
could have functions, which are used by mysql-container, mariadb-container, and postgresql-container.

See files:

• https://github.com/sclorg/mariadb-container/blob/master/test/test-lib-mysql.sh
• https://github.com/sclorg/mysql-container/blob/master/test/test-lib-mysql.sh
• https://github.com/sclorg/postgresql-container/blob/master/test/run-openshift-local-cluster